Earlier Puberty: Age 9 or 10 for Average U.S. Boy

Oct. 20, 2012 -- American boys are starting puberty up to two years earlier than decades ago, new data show.

Boys are entering puberty at an average age of 10 among whites and Hispanics, and at an average age of 9 among African-Americans. About a third of boys start to mature sexually up to two years earlier than average.

The findings come from a study of 4,131 boys examined at doctors' offices across the U.S. by Marcia Herman-Giddens, DrPH, and colleagues. The same team found a year-earlier shift in first puberty for girls.

"Contrary to what has been the popular belief of medical professionals as well as parents, boys are changing earlier, just as girls do," says Herman-Giddens, adjunct professor of public health at the University of North Carolina at Chapel Hill.

While earlier sexual maturity now is common, Herman-Giddens does not think it is necessarily healthy.

"Earlier puberty is a very important public health indicator. It is like the canary in the coal mine," she says. "Now the age of first puberty is getting lower and we have to ask why. And it is obviously important, for medical as well as social and psychological reasons, for parents and providers to know what is going on."

It's not clear why there's been a shift toward earlier puberty. The new study does not provide answers to this question.

"I suspect this is a combination of environmental factors: things like overweight, junk food, too much computer and screen time," Herman-Giddens suggests. "I want there to be questions about whether this is healthy and why this is happening."

First Signs of Puberty

Herman-Giddens and colleagues looked for two signs that boys had begun puberty: sparse pubic hair growth and an increase in the size of the testicles to 4 millimeters (at full sexual maturity, the testicles measure 25 millimeters).

Individual boys (and girls) vary widely in the time it takes them to develop from the earliest stage of puberty to full sexual maturity. Herman-Giddens says that while some boys fully mature in as little as two and a half years, others may take as long as six years. Those who start puberty soonest aren't always the first to finish.

In the Herman-Giddens study, the average age of full sexual maturity for boys of all races and ethnicities was about the same -- late in the 15th year. About a third of these boys reached full maturity two years sooner. But those who started earliest were not always the first to fully mature.